A full Elgg package that includes the framework and a core set of plugins
is available under version 2 of the GNU General Public License (GPLv2).
We also make the framework (without the plugins) available under the MIT license.

The following answers are provided as a convenience to you; they are not legal counsel.
Consult with a lawyer to be sure about the answers to these questions.
The Elgg Foundation cannot be held responsible for decisions you make based on what you read on this page.

We believe you can, since plugins typically depend only the core framework and the framework is available under the MIT license.
That said, we really recommend you consult with a lawyer on this particular issue to be absolutely sure.

Note that plugins released via the community site repository must be licensed under a GPLv2-compatible license.
They do not necessarily have to be GPLv2, just compatible (like MIT).

Can we build our own tool that uses Elgg and sell that tool to our clients?¶

Yes, but then your clients will be free to redistribute that tool under the terms of the GPLv2.